The invoice number box sits at the top of the form, next to the issue date. A new business starts at 1001 - nobody needs to know they are your customer number one - and every invoice after that takes the next number automatically. Quotes have their own sequence starting at QUO-1001.
Type a different number in the box if your accounts already run a sequence; DocMason carries on from whatever you set. Clear the box and no number prints on the invoice, the PDF or the online copy.
You can also just tell the AI creator: "make this invoice 2050" or "remove the invoice number".